As a drive system capable of reducing the fuel consumption of an engine there is known a hybrid vehicle which utilizes a motor drive force.
Various types of hybrid vehicles such as series type and parallel type have been proposed, among which is included a series-parallel hybrid type using two motors and one planetary gear. For example, in Japanese Patent Laid-open No. Hei 7-135701 there is disclosed a method wherein a driving force of an engine is inputted to a planetary gear and control is made by a generator so that a vehicle is driven with a driving force obtained from an output shaft of the planetary gear. While a part of energy of the engine is generated by a generator, a driving force is assisted from a motor connected to the output shaft to drive the engine always in a high torque region of a high efficiency, and at the same time a speed change function can be attained.
The same principle as above is also described in Japanese Patent Laid-open Nos. Sho 49-112067 and Sho 58-191364.
According to the methods disclosed therein, power is generated by a generator and a driving force is obtained by a motor for realizing a speed change function, so that there occurs an electrical energy loss. Consequently, there occurs reduction in efficiency by an amount corresponding to the electrical energy loss with respect to the whole of the vehicle concerned despite the engine can be driven always at a highly efficient operating point.
The present invention has been accomplished in view of the above-mentioned problems and it is the first object of the invention to provide a small-sized transmission constituted by small-sized motors, thereby realizing a stepless speed change function which permits vehicular operation in a region of a high engine efficiency while minimizing an electrical energy loss.
It is the second object of the present invention to provide a vehicle capable of being driven in a region of a high engine efficiency while minimizing an electrical energy loss.